An exhaust gas recirculation system for diesel engines is known from U.S. Pat. No. 5,671,600 A. A particulate filter, to which an exhaust gas recirculation conduit is connected, is provided at the exhaust gas outlet and/or at the exhaust gas conduit. Upstream of a charge-air compressor the exhaust gas recirculation conduit opens into the intake conduit. A valve for modifying the exhaust gas mass stream is provided inside the exhaust gas recirculation conduit. The particulate filter is used to protect the charge-air compressor and/or the charge-air cooler against pollution.
A particulate filter in the main exhaust gas stream of a gasoline engine is known from DE 10 2012 021 882 A1. Unlike in the case of a diesel engine, the particulate filter regenerates, to the greatest possible extent, without additional active measures, i.e. the particulate matter (soot particles with accumulated or enclosed hydrocarbons) retained until then burns under normal gasoline engine margin conditions primarily to form CO2 and H2O respectively. For this purpose, sufficiently high exhaust gas temperatures of more than 500° C. as well as oxygen are required for combustion. This exhaust gas temperature is reached soon after starting the engine in an extremely wide variety of operating ranges of the gasoline engine. Since the vast majority of gasoline engines are operated stoichiometrically, the oxygen content in the exhaust gas can be too low to completely burn off the particulate matter contained in the filter. In this case, standard overrun cut-off phases of the gasoline engine help during transient operation, during which as a result of the elimination of the load requirement by the driver the injection of the engine is cut off for consumption reasons.
Since the engine acts as an engine brake and reduces the vehicle speed when the gear is engaged, clean air is flushed through the exhaust system by the dragged engine. This unburnt air encounters the particles previously heated by the engine exhaust gas in the particulate filter.
If the temperature is sufficient, these particles catch fire and burn, forming gases which can escape through the particulate filter, resulting in the particulate filter being cleaned.
Additional measures such as a secondary air system in order to actively achieve the regeneration of the particulate filter by means of excess air are known from DE 10 2013 003 701 A1, DE 10 2013 008 426.1 and WO 2008 127 755 A2.